Detection of voltage potentials and changes to the internal ionic environment of cells can be useful in monitoring bioactivities of cells. For example, many cells experience significant changes in internal calcium ion (Ca2+) concentration in response to binding of a ligand to a G-protein receptor. In another aspect, certain cells experience large changes in voltage potential across membranes, e.g., in response to contact with neurotransmitters at a synapse. Such cellular changes are responsible for important functions in cells and can be indicative of the health, function, or development processes of the cells.
Genetically encoded calcium indicators (GECI) have been developed to allow general monitoring of the Ca2+ concentration in cells. See, e.g., Looger (U.S. 2012/0034691), wherein a calmodulin peptide sequence is combined in a construct with a green fluorescent protein (GFP) reporter. Conformational changes in the calmodulin peptide in response to Ca2+ binding changes the efficiency of the GFP fluorescence, resulting in a change in the emissions profile and intensity. Cells transformed with the GECI can be monitored generally for changes in internal Ca2+ concentration, e.g., in response to signaling ligands or inhibitors. Also see Griesbeck (U.S. 2009/0035788) wherein a FRET donor and acceptor are separated by a troponin peptide sequence, resulting in a fluorescence change on binding of Ca2+. However, such GECIs are limited in their resolution of signal, limited in ability to penetrate multi-cell/3D structures, and in the range of available applicable cell types. Typically, the old art systems are directed to two dimensional microscopic detection of signals from a cell monolayer.
In many cases, signal transduction studies are carried out in cell types that are not representative of the actual cells of interest. For example, researchers may be limited to studying signaling agents and potential therapeutics in rodents or immortal cell lines in vitro, which often provide results not repeated in human cells, or clinical patients. For example, researchers can create host cells for study by introduction of oncogenes to primary cell lines, e.g., with differentiation to a cell type of choice. However, after this process history, the cells cannot be relied on to respond normally on contact with bioactive agents. Weiss (U.S. Pat. No. 7,101,709) discloses methods of preparing multipotent mouse neural stem cells. The cells could be differentiated and transplanted to somewhat immunoprivileged CNS locations. Again, the cells would be non-representative for many studies, and signal detection is limited to immunochemical means.
In view of the above, a need exists for model cell systems representative of cells and tissues existing in live animal systems of interest. It would be desirable to have sensor peptide constructs that can be targeted to specific intracellular locations. Benefits could be realized if systems were available allowing three dimensional signal detection in mock tissues of representative cells in vitro. The present invention provides these and other features that will be apparent upon review of the following.